1. Field of the Invention
The present invention relates first of all to a method for measuring the propagation time .tau..sub.t of a wave between a transmitter-receiver of said wave, and a reflecting obstacle, in which:
said wave is transmitted modulated in amplitude by means of a periodic modulation signal,
the amplitude of said wave is detected after propagation, reflection and reception, for obtaining a detected signal,
a periodic reference signal is generated of the same period as said modulation signal and phase shifted with respect thereto by a phase shift corresponding to a delay time .tau.
the average value of the product of said detected signal and said reference signal is calculated,
said phase shift is controlled so that said average value is zero, and
said propagation time .tau..sub.t is calculated from the delay time .tau. thus obtained.
Such a process is used in telemetry or range finding, for determining the distance between the transmitter-receiver and the reflecting obstacle from the measured propagation time .tau..sub.t and the speed of the wave.
2. Description of the Prior Art
A method of the above type is already known, for example from the U.S. Pat. No. 3,752,582. In this patent the modulation signal is a sinusoidal signal of pulsation .omega. and the reference signal is formed by the modulation signal delayed by the delay time .tau.. In this case, when .tau. corresponds to cancellation of the average value of the product of the detected signal and the delayed modulation signal, it is easy to show that we have: EQU .tau..sub.t =.tau.-.pi./2.omega.+2k .pi./.omega.
where k is any integer.
To get over the indetermination of 2k.pi./.omega. a pulsation .omega. of the transmitted signal is chosen so that the term EQU 2.pi./.omega.
is greater than the maximum propagation time (.tau..sub.t).sub.max to be measured, namely: EQU 2.pi./.omega.&gt;(.tau..sub.t).sub.max
The maximum propagation time (.tau..sub.t).sub.max to be measured is naturally related to the maximum distance to be measured L.sub.max by the relationship: EQU (.tau..sub.t).sub.max =2L.sub.max /v
where v is the speed of the wave in the propagation medium.
In this case, the accuracy of the measurement is fairly low, for it is difficult to estimate very small phase shifts, especially in the presence of a noise which affects the detected signal and because a phase shift error leads to a distance error all the greater the smaller the pulsation.
To overcome the above drawback, a method, described in the article by D. E. Smith "Electronic distance measurement for industrial and scientific applications" in Hewlett Packard journal, vol. 31, No. 6, June 1980, pages 3-11, Palo Alto, U.S., overcomes the ambiguity by proceeding by successive estimations. For that, several measurements are made successively on signals S.sub.o, S.sub.1, . . . S.sub.i, and S.sub.I having pulsations .omega..sub.o, .omega..sub.1, . . . .omega..sub.i, . . . and .omega..sub.I, such that the pulsations .omega..sub.1, . . . , .omega..sub.i, . . . and .omega..sub.I are increasing and multiples of the pulsation .omega..sub.o. Under these conditions, the last measurement made at the highest pulsation gives the best resolution. But, this implies several successive measurements which take time. In addition, if between two successive measurements the distance to be measured varies too quickly, the result may be erroneous. The present invention aims at overcoming the above drawbacks by providing a method which, in a single measurement, allows the value of the distance to be measured to be obtained without ambiguity, even if it is great, with a resolution equal to that which is obtained in the prior art methods with a high pulsation signal.